xen: adopt .deinit_pdata and improve timer handling

xen: adopt .deinit_pdata and improve timer handling

The scheduling hooks API is now used properly, and no
initialization or de-initialization happen in
alloc/free_pdata any longer.

In fact, just like it is for Credit2, there is no real
need for implementing alloc_pdata and free_pdata.

This also made it possible to improve the replenishment
timer handling logic, such that now the timer is always
kept on one of the pCPU of the scheduler it's servicing.
Before this commit, in fact, even if the pCPU where the
timer happened to be initialized at creation time was
moved to another cpupool, the timer stayed there,
potentially inferfering with the new scheduler of the
pCPU itself.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Reviewed-and-Tested-by: Meng Xu <mengxu@cis.upenn.edu>
Acked-by: George Dunlap <george.dunlap@citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>